In the field of burglar alarms and intrusion detectors, the use of mercury switches for sensing a tilt condition or acceleration is well known. In these devices the switch usually comprises a drop of mercury and electrodes of equal length enclosed in a capsule with a non-corrosive atmosphere. Any sudden motion imparted to the device or change in inclination causes the mercury to run into the electrodes, closing the switch and actuating the alarm.
A major drawback to these devices is that the switch itself is not readily adjustable in sensitivity. Thus a great many false alarms may be sounded, at great expense and annoyance, or the actual alarm condition may not be sensed, at greater expense and disappointment. To remedy this situation, a great amount of effort has been expended in the prior art to design electronic systems which can discriminate with a high degree of certainty between false and true alarm indications. These systems are complex and expensive, and accordingly temperamental and difficult to service.
Another failing of the prior art devices is that in general the switches are sensitive to motion imparted from any direction. Thus a prior art device protecting a pane of glass may be actuated by the backfire of a passing automobile as well as the breaking of the pane. In another instance a mercury switch device employed to detect the motion associated with broachment or scaling of a fence may be falsely actuated by a gust of wind. These failings are both due to a lack of directional sensitivity of the prior devices.